The invention relates to a cleaning device which removes any residual toner from an electrophotographic copying machine.
In an electrophotographic copying machine, it is recognized that any residual toner which remains on a photosensitive drum without being transferred onto a copy sheet, subsequent to a transfer of a toner image thereto, must be removed by a cleaning operation, prior to the formation of a new electrostatic latent image thereon. A variety of cleaning devices have been proposed to this end, including a fur brush, a blade or a magnetic brush. In an electrophotographic copying machine including a developing unit which utilizes a dry developer as a toner to develop an electrostatic latent image formed on a photosensitive drum, it is preferred to recover any residual toner on the drum into the developing unit for re-use. Such recovery is disabled with a cleaning device which utilizes a fur brush, since a residual toner removed by the fur brush is generally trapped by a filter as it is conveyed on an airstream, produced by an air suction mechanism, to be disposed. Additionally, the arrangement is complex and bulky in size since a suction mechanism and a filter must be provided, resulting in a disadvantageous increase in the size of the entire copying machine. A cleaning device of blade type employs a blade formed from an elastic material such as urethane and which is held in contact with the photosensitive drum for the purpose of cleaning the drum. This latter arrangement simplifies the construction and reduces the size of the copier machine, and any residual toner on the drum can be collected by the blade for recovery into the developing unit for re-use. However, in the prior art practice, a toner vessel is provided to receive residual toner collected by the blade in order to prevent a free fall from the drum, and the vessel is manually discharged into the developing unit in a periodic manner. Alternatively, toner received in the toner vessel is automatically conveyed into the developing unit by separate toner conveyor means including a flexible screw or the like. In the former instance, the maintenance represents a troublesome operation while in the latter, the arrangement becomes expensive, and the toner conveyor means is complex in construction.
Where a magnetic brush is used in a cleaning device, the latter may also serve as a developing unit. In such instance, any toner that is removed can be directly carried into the developing unit, thus advantageously dispensing with the removal of the removed toner to the exterior of the copying machine, a periodic maintenance which recovers the toner into the developing unit, or a complex arragement for automatically returning the removed toner into the developing unit. In this manner, the magnetic brush cleaning device is advantageous in reducing the cost and the maintenance of the arrangement. However, when the developing unit also serves as a cleaning device, the location of the developing unit which is normally chosen, namely, intermediate a station where a latent image is formed and a transfer station to transfer a toner image, makes it impossible to effect a cleaning and a developing step simultaneously as is done in an electrophotographic arrangement which is provided with a devoted cleaning unit located intermediate the transfer station and the latent image forming station. Accordingly, where it is desired to produce a plurality of copies in succession, at least two revolutions of the photosensitive drum are required to produce a single copy. In addition, the peripheral length of the photosensitive drum must be greater than the length of copy sheets of the maximum size. Accordingly, as compared with an electrophotographic machine having a devoted cleaning unit, the machine with the combined developing and cleaning unit must be designed to operate with a higher process speed in order to achieve the same rate of producing copies. In addition, the constraint on the configuration of the photosensitive drum disadvantageously results in an increased overall size of the arrangement.
FIG. 1 is a schematic view of one exemplary electrophotographic copying machine provided in the prior art in which a combined developing and cleaning unit is used. In FIG. 1, as a copying operation is initiated, a photosensitive drum 1 begins to rotate in a direction indicated by an arrow a, and is uniformly charged by a corona charger 2. An original to be copied is placed on a movable receptacle 3 and is fed in a direction indicated by an arrow b. As the original moves, it is irradiated by an illumination lamp 4 cooperating with a reflecting mirror 5, and an optical image of the original is projected onto the drum 1 by a fiber lens array 6, thus forming an electrostatic latent image corresponding to the image of the original in receptacle 3 on the drum 1. A developing unit 7 which utilizes dry developer includes a magnetic brush roller 8, which developes the latent image with the toner, and the resulting toner image is brought into superimposed relationship with a copy sheet, which is fed from a sheet cassette 9 by a pickup roller 10 and a pair of feed rollers 11. A corona charger 12 acts to transfer the toner image onto the copy sheet. The copy sheet is separated from the drum 1 under the action of another corona charger 13 which effects a separating action. Subsequently, the copy sheet carrying the toner image is passed between a pair of fixing rollers 14 where the toner image is fixed. Subsequently, it is delivered onto a copy tray 16 by a pair of delivery rollers 15. The cleaning step which removes any residual toner from the drum 1 after the toner image has been transferred onto the copy sheet takes place by a continued rotation of the drum 1 through one revolution. Initially, the charge is removed from the drum 1 which may have any residual toner on its surface, by a uniform exposure to illumination from a neutralizer lamp 17 and any residual toner is recovered into the developing unit 7 as a result of a rubbing contact of the drum with the magnetic brush roller 8 in the developing unit 7. Subsequently, when the drum 1 from which any residual toner has been removed rotates to a position corresponding to the charger 2, another copying operation is initiated when a next original is set in position. Thus, it will be seen that it is necessary that the drum 1 rotates through two revolutions per copy where a plurality of copies are to be produced in succession. Furthermore, since the developing unit 7 performs both the developing and the cleaning operation, the peripheral length of the drum 1 must be greater than the length of copy sheet the maximum size employed in the copying machine. Because of these requirements, the peripheral speed of the drum 1 must be increased as compared with an electrophotographic copying machine having a devoted cleaning unit in order to achieve the same rate of producing copies per unit time, and accordingly a higher duty is imposed upon the chargers 2, 12, 13, the lamp 4, the developing unit 7 and the fixing rollers 14. When producing a copy of a reduced size, it is possible in an electrophotographic copying machine having a devoted cleaning unit that the forward and reverse stroke of an original receptacle or scanning optics be changed in accordance with copy size to increase the number of copies produced per unit time. However, in the arrangement shown in FIG. 1, the number of copies produced per unit time cannot be increased for copies of reduced size. It is possible to increase the number of copies produced per unit time by increasing the speed of the original receptacle and scanning optics during their reverse stroke in the arrangement having a devoted cleaning unit, but such measure cannot be taken in the arrangement of FIG. 1. Also, in the arrangement having a devoted cleaning unit, the diameter of the photosensitive drum can be reduced concomitant with a requirement that associated devices can be arranged around the drum. However, in the arrangement of FIG. 1, the drum diameter cannot be reduced below a value which provides a peripheral length corresponding to the maximum size i.e., length of copies to be produced. With respect to the cleaning function, it is necessary in the arrangement of FIG. 1 that a supplementary measure be taken as by increasing a developing bias voltage to assure a satisfactory cleaning performance when the unit aso serves as a cleaning unit.
An electrophotographic copying machine having a combined developing and cleaning unit is used in practice despite a number of drawbacks as mentioned above, for the reason that a devoted cleaning unit which is complex and which involves difficulty in maintenance can be dispensed with, while affording an advantage that any toner removed by the cleaning operation can be entirely utilized again.